This invention relates in general to superconducting magnet systems and, more specifically, to a combined persistent switch and quench heater for use with large superconducting magnets.
Over the last few years, steady progress has been made in developing superconducting magnet technology for a wide range of applications, such as superconductive magnetic energy storage and load-balancing, magnetohydrodynamic marine propulsion systems, magnetic levitation rail propulsion, large high energy physics particle detector magnets, magnetic fusion coils and the like. These applications require the use of very high currents, with correspondingly large leads penetrating through the magnet refrigeration system for carrying the currents into and out of the superconducting magnet coil. Typically, with helium refrigeration, a well designed lead pair will have a helium liquifaction load of approximately 3 liters per hour per kiloamp. For high current applications in the 50-300 kA range, the refrigeration power requirements associated with the lead heat loads can become enormous, particularly in terms of power needed for the room temperature helium compressor. In mobile magnet systems, such as magnetic levitation rail systems and magnetohydrodynamic submarine propulsion systems, reductions in refrigeration requirements would result in reduced weight and/or increased range and performance. Thus, the ability to remove the leads during phases other than the charging and discharging periods would be highly desirable.
Any integrated removable lead scheme must provide the capability of shorting the main coil when the leads are removed. Mechanical switches connecting the coil terminals could be used, but their use would result in a large heat leak to the low-temperature refrigeration system, defeating the purpose of having the removable leads.
Persistent switches are commonly used to short the leads of small superconducting magnets, as described, for example, by M. N. Wilson, "Superconducting Magnets", Clarendon Pres, Oxford, England, 1983. A persistent switch is a non-inductive coil of superconducting wire whose temperature and resistive state can be controlled by means of a heater. When the heater is off, the wire comprising the switch is stable and in the superconducting state. This corresponds to the switch being closed and shorting the coil. Heat input to the switch coil winding drives the superconductor in the switch normal and opens the switch. Opening the switch provides the resistance which creates the potential across the coil necessary for charging and discharging.
Small, high current density, superconducting magnets operate at steady-state and are self protecting, i.e., normal zones propagate fast enough that the stored magnetic energy is dissipated over a sufficient volume of winding so that the conductor does not overheat. Larger high-field superconducting magnets have in the past required either an external dump resistor with permanent, high heat leak power leads or a system of heaters with uninterruptible power supplies and robust power leads to drive the winding normal and prevent damaging temperature excursions in winding hot spots.
Thus, there is a continuing need for a switching system for permitting the charge/discharge leads of a large superconducting magnet coil to be removed and the coil terminals to be shorted to provide an uninterrupted current path for persistent mode operations. This shorted path must be able to carry the full coil current, be of low resistance, and have the capability to act as a switch so that the magnet can be charged and discharged when the leads are connected. The switching system must also be absolutely reliable with very low charging and discharging losses.